Linear light polarizers in general owe their properties of selectively passing radiation vibrating along a given electromagnetic radiation vector and adsorbing electromagnetic radiation vibrating along a second given electromagnetic radiation vector to the anisotropic character of the transmitting medium. Dichroic polarizers are linear polarizers of the absorptive variety; that is they owe their polarizing capabilities to the vectorial anisotropy of their absorption of incident lightwaves. Light entering a dichroic medium encounters two different absorption coefficients, one low and one high. The emerging light vibrates predominantly in the direction of low absorption.
The most widely used type of synthetic polarizer, which is the polarizer to which the present invention is directed, is the polyvinyl alcohol-iodine complex polarizer. It consists of linear polyiodide contained within a polyvinyl alcohol helix. By orienting the polyvinyl alcohol matrix unidirectionally the transition moments of the absorbers are also so oriented and the material becomes visibly dichroic.
Polyvinyl alcohol film polarizers generally comprise a plastic support which may be any suitable isotropic material, and preferably cellulose acetate butyrate. The support gives the film dimensional stability and additionally serves as a cover or protective element. It must naturally be transparent.
The manufacture of iodine stained dichroic light polarizing elements, which involves stretching polyvinyl alcohol and subsequently dyeing the material with a dichroic stain containing iodine, is well known and is disclosed, for example, in U.S. Pat. No. 2,237,567 of Edwin H. Land issued on Apr. 8, 1941. In accordance with that patent it is disclosed that a cast sheet or film of polyvinyl alcohol is first formed from an aqueous solution of the material. The dried cast sheet is then heated to a temperature at which it can be extended by stretching, preferably in a moist atmosphere. It is further disclosed in that patent that the stretched sheet may be bonded to a supporting sheet, as has been discussed above. After the stretched sheet is cooled, a dichroic sheet may be applied to one or both surfaces of the sheet.
In a later patent issued to Alexander Thomas on May 15, 1945, U.S. Pat. No. 2,375,963, an improvement in the process of making an iodine stained polarizer is described and essentially comprises washing the polarizer after the step involving staining with iodine. This results in removing uncombined iodine and forming a more stable product.
A major improvement in the above light polarizing element is explained in U.S. Pat. No. Re. 23,297 issued on Nov. 28, 1950 to Mark Hyman, Jr. et al. That improvement comprises a protective surface layer on the iodine stained polyvinyl alcohol light polarizer. That surface layer comprises an ester of polyvinyl alcohol, the ester being of a polybasic acid or a derivative of such an acid, particularly an inorganic polybasic acid, and more particularly boric acid, thereby providing a hybrid form of polyvinyl alcohol-polyvinyl borate. This borating step is disclosed to provide greatly improved stability to the light polarizer not only against heat but also against moisture and ultraviolet radiation. It is accomplished by treating the stained polarizing element with a boric acid solution. Specifically the ester formed on the surface of the light polarizer is believed to be polyvinyl orthoborate. The esterification reaction may be accompanied by some decrease in the degree of molecular orientation and loss of stain in the reacted layer. This in turn may cause a loss in the dichroism of the sheet and a deterioration in transmission running as high as from 5 to 10 percent of incident light. The predominant color of the sheet also changes toward the blue-that is the sheet becomes less efficient in its overall blue adsorption.
The diminution in the blue dichroic absorption or darkening of borated polarizer can be prevented by eliminating the iodine from so much of the surface layer of the sheet as will be penetrated by the boric acid solution. One way of eliminating this iodine is to wash the sheet with water after staining and before treatment with the boric acid solution. A superior technique for avoiding the loss of blue absorption involves incorporating a high level of potassium iodide in the borating solution. Apparently this reinforces the dichromophore responsible for absorption in the blue range. After the material has been borated and dried, excellent blue dichromophore stability and a high level of blue absorption is maintained. However, a significant instability which results in a drop-off in red light adsorption results on heating, thereby producing a significant "red leak" which is particularly noticeable when two such polarizers are in the crossed position. The "red leak" is quite noticeable, particularly in polarizers with low light leakage at the lower end of the spectrum, i.e., the blue end.